While catalytic converters are well known for reducing oxides of nitrogen (NOx), and oxidizing hydrocarbons and carbon monoxide from automobile exhaust, these reactions typically take place after the catalyst has attained its light-off temperature, at which point the catalyst begins to convert the hydrocarbons to harmless gases. The typical catalytic light-off time for most internal combustion engine systems is around 50 to 120 seconds (generally in the temperature range of 200.degree.-350.degree. C.), with the actual catalytic light-off time for any system depending on a number of factors; including, the position of the catalyst relative to the engine, the aging of the catalyst, washcoat technology, as well as the noble metal loading. Seventy to almost ninety five percent of hydrocarbon emissions from automotive vehicles are emitted during this first minute, or so, of "cold start" engine operation. Without additional measures large amounts of hydrocarbons are likely to be discharged into the atmosphere during this period. Environmentally, increasing the effectiveness of automotive emission control systems during cold start, so that the amount of hydrocarbons discharged into the atmosphere during cold-start are kept at extremely low levels, has become increasingly important.
Various methods have been proposed for meeting the stringent hydrocarbon emission standards during cold start including, the use of electrically heated catalysts (EHCs), the use of molecular sieve structures (hydrocarbon adsorbers) to adsorb and hold significant amounts of hydrocarbons until the converter has attained its light-off temperature, as well as combinations of both. Recently, an improved in-line exhaust system was in U.S. application Ser. No. 08/484,617 (Hertl et al.); assigned to the instant assignee, and herein incorporated by reference. Specifically, Hertl et al. discloses an in-line exhaust system having a main catalyst, a housing downstream of the main catalyst having an inlet and an outlet end, and having disposed therein a molecular sieve structure for adsorbing hydrocarbons. The molecular sieve structure exhibits: (1) a first region forming an unobstructed or substantially unobstructed flow path for exhaust gases of an exhaust stream; and, (2) a second, more restricted flow path or region adjacent the first region. Furthermore, the exhaust system includes a burn-off catalyst disposed downstream from the adsorber having a light-off temperature. The system also includes a diverting means disposed in the housing for passing secondary air into the housing; the flow pattern of the secondary air directs a portion of the exhaust gases of the exhaust stream through the second region of the adsorber prior to the main catalyst attaining its light-off temperature.
Although, the system of Hertl performs better than earlier exhaust systems, environmental concerns and legislation drafted to meet those concerns continues to lower legally acceptable hydrocarbon emission standards, e.g., the California ultra-low emission vehicle (ULEV) standards. Notwithstanding the foregoing developments, work continues to discover improvements to existing systems and to provide new systems capable of meeting the stricter exhaust emission standards.
One such improvement is disclosed in copending, coassigned application, U.S. Ser. No. 08/578,003 (Brown et al.) wherein it discloses an exhaust system comprised of the following: (1) a honeycomb structure having an inlet and outlet end disposed in a housing and possessing a first substantially unobstructed flow region, a second more obstructed flow region adjacent the first region; and, (2) a fluidics apparatus disposed in the exhaust stream proximate to the first region for creating a negative flow zone within the first region. The fluidics apparatus of Brown includes a source of a diversion fluid, typically air, and a diverter body for diverting the diversion fluid, both of which combine to result in a negative flow zone which aids in diverting the exhaust gas away from the first flow region toward the second flow region.
Although this system provides improved performance for substrates possessing two flow paths/regions, the resulting flow characteristics for this system when the fluidics apparatus is functioning, e.g., under adsorbing conditions, are not ideal. For example, the system of Brown, although an improvement over exhaust systems to date, produces an exhaust or air flow pattern, under diverted conditions which, generally exhibits a high flow volume in the cells proximate to the center hole and a significantly decreased flow volume in those the peripheral cells nearer the edge of the substrate. As a result, work continues to discover improved fluidics systems for use with exhaust systems and accordingly, it is one of the objectives of the instant invention to provide an engine exhaust system exhibiting a more uniform diverted exhaust flow.